Coal used in a thermal electric power plant, an iron work or the like is usually stored as a pile stacked in an outdoor yard. The coal thus stored produces heat by a reaction with oxygen in the air to sometimes cause spontaneous ignition. In particular, a low-rank coal has high oxidation reactivity because of its porous form and easily produces heat. Therefore, a method of preventing spontaneous ignition by water spraying or the like onto a pile has been generally used. However, since this method requires periodic water spraying, an efficient spontaneous ignition prevention method has been desired.
In such circumstances, as techniques for preventing spontaneous ignition of a coal pile, there have been proposed a method of covering a pile surface with a resin or the like (see JP-A-5-230480 and JP-A-2000-297288) and a method of spraying a surfactant containing a radical trapping agent or an oxygen trapping compound (see JP-A-2001-164254). However, since the above-mentioned respective methods require the resin, the radical trapping agent or the like, there is a concern about a rise in cost.
On the other hand, a production method for obtaining a modified coal from a low-rank coal (porous coal) having a high water content rate and a low calorific value has been developed (see JP-A-7-233383). In this production method, the porous coal is first ground into a granular form, and thereafter, mixed with a mixed oil containing a heavy oil component and a solvent oil component to obtain a raw material slurry. Next, after preheated, the raw material slurry is heated, thereby proceeding with dehydration of the porous coal and impregnating the mixed oil in pores of the porous coal to obtain a dehydrated slurry. Then, after a modified porous coal and the mixed oil are separated from the dehydrated slurry, the modified porous coal is dried (deliquored). The dried modified porous coal is cooled and molded as desired. By this production method, the heavy oil adheres to the inside of the pores of the porous coal, with a decrease in the water content rate of the porous coal, and thus the modified coal having a high calorific value can be obtained.
The modified coal obtained by the above-mentioned production method is molded into briquettes, from the viewpoint of workability including a transport work or from the viewpoint of suppressing dust generation. When the briquettes are stored as a pile, air permeability of the pile is high because it is composed of the briquettes having the same shape. When coal having relatively high oxidation reactivity is piled, or when the height of the pile is increased, a temperature rise of the pile occurs in a relatively short period of time. Accordingly, in such a modified coal, a storage technique which is especially less likely to cause spontaneous ignition is desired.